This phenomenon occurs when you drop a turd and the impact of the poo on the water sprays your butt-cheeks with a mocha coloured combination of dookie and toilet water. Most of the time collateral damage occurs when slaying diarrhea, since the glugy like substance enters the water at a fast rate thus creating alot of spray. Furthermore collateral damage occurs when taking a huge, but clean and fast individual log that enters the water in such a manner that it creates a fluctuation in water surface sending water particles to your ass. This is a weird sensation and is most effectively avoided by laying a thin layer of toilet paper in the toilet giving your beazley a smooth landing surface. Also this phenomenon can be avoided by dropping a grogan, which is a marvel if achieved since only the strong and the wise can do this.

"My ass is wetter than an indonesian swamp after that collateral damage"

Refers to getting two kills with one shot on C.O.D. i.e. you aim at a guy with a sniper rifle and shoot, not knowing there is another person behind him. The bullet goes through the first guy and kills the second guy thereby giving you two kills with your one shot.

When someone is having major issues in their life that are totally unrelated to you but for some insane reason they are taking it out on you. You are now collateral damage in a fight that they should be having with someone else.

When Julie caught Tommy cheating on her she became convinced that all her friends were sleeping with him. She exploded on everyone and we were all collateral damage.